1. Field of the Invention
This invention relates to scanning antennas, and more particularly to antennas in which a scanning action is achieved by rotating an anisotropic medium relative to an input signal.
2. Description of the Related Art
Present scanning antennas are quite costly and technologically sophisticated. They can be divided into two main classes: mechanically rotating systems and phased array systems. The systems that use a mechanical rotation to achieve a scanning movement are generally bulky, and the rotating mechanisms require periodic maintenance. Scanning phased array antenna systems, on the other hand, typically use ferrite and diode phase shifters that are high cost precision items requiring considerable amounts of power to operate.
Another system for which a prototype has been demonstrated employs a series of parallel metal plates that rotate about an eccentric axis within a slotted waveguide. In the course of their rotation the plates move in and out of the field lines within the waveguide; this causes the speed at which an input beam propagates through the waveguide, and accordingly its wavelength, to vary. The waveguide slots are spaced periodically with respect to the limits of the wavelength range such that the signal radiated out through the slots is steered back and forth through a scanning angle as the plates continue to rotate. This type of system is expected to be costly and difficult to manufacture in a mass production process because the dimensional precision of the rotating core structure and of its axial positioning are highly critical, and the system is quite lossy and has a high power consumption. This type of system is described in Hansen ed., Microwave Scanning Antennas, Chap. 1 by Kummer, Academic Press, 1966, pages 64-65.
Another approach to the achievement of a scanning antenna is described in U.S. Pat. No. 3,631,501 to Busher. In this patent, metallic particles are dispersed within a liquid optically isotropic medium. (An optically isotropic medium is one in which the index of refraction is independent of both the direction in which light propagates through the medium, and of the light's state of polarization.) Unfortunately this type of antenna did not work well because the metallic particles were not stable within the liquid, and tended to settle out. Also, an unduly large electric field (greater than 10.sup.5 V/cm) was needed to obtain the 1 microsecond switching speeds mentioned in the patent.
It would be very desirable to have a simple scanning antenna system with low maintenance requirements, compactness, ruggedness, low cost and low power consumption for millimeter range radar applications such as automobile collision avoidance, cruise control radar systems and aircraft ground approach radar systems. However, there is no known radar antenna system that is compact and economic enough to satisfy all of these requirements.